RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v4]

[Xiaohong Gong]

On Wed, 25 Jun 2025 08:06:40 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>> Maybe the NEON `tbl` can also be generated for SVE2 when `length_in_bytes == 16 && length_in_bytes < MaxVectorSize`. This is a special partial version for SVE2.  As a summary, The match rule's predicate will be:
>> 1) NEON: UseSVE < 2 || (length_in_bytes < 16 || length_in_bytes < MaxVectorSize)
>> 2) SVE: UseSVE ==2 && (length_in_bytes >= 16 && length_in_bytes == MaxVectorSize)
>> 
>> Seems this will make predicate or code here more complex. Advantage is this op with 128 vector shape on a SVE2 256 or larger size machine will also be intrinsified. It's not a block and change or not is up to you. We can also revisit this part once the 256-bit SVE2 machine exist in future.
>
> Thanks @XiaohongGong . The case you mention will need an SVE2 machine with MaxVectorSize >= 32B which is currently not available. I think it's better if we revisit these cases once a functioning hardware is available. Shall I add a comment here as a reminder that we need to revisit when such hardware is available?

I think it's fine without more comments. We can get the information from rules or from performance results.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2166090846